Opportunity is the crux of the American idea. Opportunity is what the New World has always represented: struggle, risk, self-determination, and the hope of spiritual and material progress. Even now, to new immigrants, that or something like it is the pull—and for them at least, it is no false promise. If you move to America, you move up, and this is true whether you are rafting across the Rio Grande or negotiating the hazards of the H1B visa program. British emigrants (I am one) are fond of Spain and the United States. They go to Spain to retire; they come here to rise to new challenges. This lure, barely diminished after more than three centuries, has ever been an incalculable source of national strength.

But is America any longer a land of opportunity for the people born here? The evidence, such as it is, points to a surprising and dispiriting answer: no, not especially.

The idea that America is exceptional in its material opportunities is deeply lodged in the culture. For as long as the country had a western frontier with territory beyond, internal migration was just as bold a venture as crossing the ocean had been for the first settlers, and just as promising for the ambitious and self-reliant. The late-19th and early-20th centuries brought extraordinarily rapid industrial development, which nourished the American idea in a new way. Rising incomes made each succeeding generation more prosperous—and they rose so fast that people even felt more prosperous. But that phase, too, has ended. Incomes are now rising more slowly from generation to generation (and for a variety of reasons, the flattening feels worse than it is). Fewer adults today, it seems, expect their children to do better than they did. Pessimism vies with vitality for command of the national consciousness.

Much of this, no doubt, is a natural consequence of growing old. New immigrants notwithstanding, America is a middle-aged country, and striving is not a trait of the middle-aged. Still, an accumulating body of research suggests that the stiffening of America’s socioeconomic sinews is more advanced than the culture, even now, seems willing to admit; worse than the scholars who monitor it had hitherto understood; and—how shaming is this?—worse than in many older, wearier countries.

The American model has been regarded as proposing a kind of bargain. This is not Europe: Here, idleness and incompetence are sternly punished—but merit gets rewarded. Much more than elsewhere, your class background will neither prop you up nor hold you back. If you deserve to succeed, you will.

It is an inspiring, energizing offer—and still a profoundly influential one. It colors the national debate about taxes, health care, and other aspects of economic policy. But it is false advertising.

Most researchers now give America much lower marks than they used to for intergenerational economic mobility—the ease with which successive generations move up or down relative to their parents. As flaws in early postwar studies have been addressed, estimates of mobility have fallen. Before the 1990s, researchers tended to put the correlation between parents’ incomes and their children’s at around 20 percent, implying a high degree of mobility between generations. (Zero would imply no connection at all; a correlation of 100 percent would imply that parents’ incomes entirely determined the incomes of their children.) In the 1990s, using better data and techniques, experts tended to put that figure at about 40 percent. Recent estimates run as high as 60 percent. The finding is not that mobility has fallen since World War II—the studies point to no clear trend. It is that as methods of measuring mobility have improved, the result, across a span of recent decades, has gotten worse. The earlier view that postwar America was an economically mobile society is less and less borne out. Perhaps it was once (before data became available to track such things accurately); but it isn’t now.